The problem of analog transmission via fiber optics, especially for Cable Television (CATV) transmission and distribution, has received strong impetus with the introduction of some AM fiber links based on novel distributed feedback (DFB) semiconductor lasers. Analog AM transmission systems and distribution networks benefit from fiber-optic transport, since the distance between electrical repeaters can be significantly increased as compared with electronic distribution networks. However, the requirements on second-order and third-order linearity are very stringent for directly modulated lasers to be used in the optical transmitters, since the presence of non-linearities causes intermodulation distortion which, in the case of CATV transmission for example, shows up as intolerable degradation of the television picture. Because of non-linearity of the optical source, the modulating signal amplitude has to be limited to small values in order to maintain the composite triple beat and second order distortion specifications under a tolerable level. A reduction of the optical source non-linearity would directly translate into the ability to increase the modulation signal amplitude while still maintaining the composite distortion specifications. An increased modulation amplitude (modulation index) is equivalent to better signal to noise ratio, i.e., the ability to distribute the analog signal over larger distances, to split the signal to more receiving sites, or to transmit more channels on the same link. The main obstacle with AM fiber transmission is how to overcome the non-linearity limitations of optical sources for analog transmission. Currently used approaches include:
1. Careful laser device selection, using distributed feedback lasers and other types of semiconductor low-noise lasers, and trying to carefully select individual devices or modify the fabrication process in order to produce a more linear response, for example preventing leakage current around the active lasing area. The problem with this approach is that currently the yield for devices with sufficient linearity is quite low, and reliable fabrication techniques for more linear laser devices have not yet been found.
2. Feedforward techniques, as described by J. Koscinski, "Feasibility of multichannel VSB/AM transmission on fiber optic links" NCTA Technical papers, Las Vegas 1987 p. 17. In these techniques, compensation of the nonlinearity is achieved by isolating the distortion produced in a nonlinear circuit and subsequently injecting the processed error back into the circuit. The disadvantages of these methods are in the requirements to use matched sources and the cost of two optical sources and the complexity of delay and gain balancing.
3. Negative feedback techniques, such as described by J. Koscinski, "Feasibility of multichannel VSG/AM transmission on fiber optic links," NCTA Technical papers, Las Vegas 1987, p. 17, rely on a photodiode to monitor the optical signal and provide the necessary feedback signal. The amount of distortion compensation depends on the feedback gain. Although the application of negative feedback is straightforward, large bandwidth requirements may create problems at high frequencies rendering this technique impractical.
An alternative to directly modulating lasers is using external modulators in conjunction with CW lasers. A directly modulated laser such as a DFB laser tends to be more sensitive to optical back reflection, and noisier than a CW laser which is modulated externally. The back reflection into the laser yields a non-linear light vs. current response when coupled with chirping effects of the directly modulated laser. The advantages of using modulators vs. lasers in terms of the non-linear intermodulation distortion have been discussed by G. E. Bodeep, T. E. Darcie, "Comparison of second and third order distortion in intensity modulated InGaAsP lasers and an LiNbO.sub.3 external modulator," Paper WK2, OFC89' Conference on Optical Fiber Communications, Houston, Tex., February 1989, where it was concluded that external modulators tend to have lower second order distortion but higher third order distortion then directly modulated lasers. However, the ability to maintain the second order distortion of a modulator at a sufficiently low level cannot be taken for granted, and depends on how close to the quadrature point the device is biased. Fabrication imperfections, temperature changes, optical damage, and the like may cause the bias point to drift away from quadrature, in which case the second order distortion becomes more substantial. In order to use external modulators effectively for analog transmission applications, the drift in the bias point (which causes second harmonic distortion) as well as the third order harmonic distortion of the device must be eliminated or greatly reduced, since prior art modulators provide adequate linearity only over a very limited range around the quadrature point.
It is generally known that external modulators have a transfer characteristic that is considerably more stable than that of lasers. Unlike the lasers' light vs. current curve, the shape of the modulators, transfer characteristic is generally unaffected by optical power, temperature, aging, and the like, although the quiescent point of operation on the fixed transfer characteristic is affected by these factors.
If means were found to extend the linear range around the quadrature point, then the modulation index of the analog information signal could be increased and the performance of analog links improved, with better signal to noise ratio, and the ability to provide longer links.
Henry A. Blanvelt, Howard L. Loboda, "Predistorter for linearization of electronic and optical signals", U.S. Pat. No. 4,992,754 is applicable to the linearization of distributed feedback lasers but is not quite applicable to the linearization of external modulators in terms of the high suppression required.
Richard B. Childs and Vincent A. O'Byrne, "Multichannel AM Video Transmission using a high power Nd:YAG Laser and Linearized External Modulator" IEEE Journal on selected areas in communication, Vol. 8, No. 7, Sept 1990, Richard B. Childs and Vincent A. O'Byrne, "Predistortion linearization of directly modulated DFB lasers and external modulators for AM video transmission", WH-6 OFC '90, San Francisco, Calif., January 1990, and Richard B. Childs and Vincent A. O'Byrne, "50 Channel VSB-AM Video Transmission Employing a Linearized External modulator", PD23-1, OFC90, San Francisco, Calif., January 1990, refer to external modulator linearization by predistortion.